Photosensitive media processors, such as Kodak X-OMAT processors, are useful in applications like the automatic processing of radiographic films for medical imaging purposes. The processors automatically transport sheets or rolls of photosensitive film, paper or the like (hereafter "film") from a feed end of a film transport path, through a sequence of chemical processing tanks in which the film is developed, fixed, and washed, and then through a dryer to a discharge or receiving end. The processor typically has a fixed film path length, so final image quality depends on factors including the composition and temperature of the processing chemicals (the processor "chemistry"), and the film transport speed (which determines the length of time the film is in contact with the chemistry).
In a typical automatic processor of the type to which the invention relates, film transport speed is set at a constant rate and the chemistry is defined according to a preset recommended temperature, e.g. 94.degree. F. (34.degree. C.), with a specified tolerance range of +/-X.degree.. A temperature control system is provided to keep the chemicals within the specified range.
Some processors use a thermowell located in a developer recirculation path to maintain a desired recommended developer chemical temperature. The thermowell has a cartridge heater inserted into one end of a hollow tubular body through which the developer is caused to flow by means of a pump. A thermistor protruding into the thermowell flow path serves to monitor the recirculating developer temperature. The duty cycle of the heater is varied, based upon data received from the thermistor, as a function of the proximity of the measured actual temperature to a preestablished developer setpoint temperature. Until the setpoint temperature is reached, a "wait" light or similar annunciator signals the user that an undertemperature condition exists. Once the setpoint temperature is reached, heating and cooling cycles are initiated, as needed, in accordance with detected temperature variations from the setpoint. Cooling may be accomplished by operation of a solenoid valve which redirects the developer through a loop in the recirculation path which is in heat exchange relationship with cooler water in the wash tank. An overtemperature limit, typically 1/2.degree. above setpoint temperature, is established as a reference to determine proper operation of the heating control system. If an actual temperature greater than the overtemperature limit is sensed, an overtemperature error is signalled. The fixer, whose temperature is less critical, may have its own thermowell recirculation path or may be maintained at a temperature close to the developer temperature by directing it in heat exchange relationship with the developer.
While processors used for radiographic image processing are traditionally configured to operate at a single film transport speed and developer setpoint temperature, new processors have been introduced which are settable as to transport speed and temperature, so the same processor can be used for multiple processing modes. A particular mode is often referred to by a shorthand designation indicative of its associated "drop time," which corresponds to the time lapse from entry of the leading edge of a film at the feed end of the processor, until exit of the trailing edge of the same film at the discharge end. Kodak uses the designations "Kwik" or "K/RA," "Rapid," "Standard," and "Extended" to refer to different user-selectable operating modes, each of which has its own characteristic transport speed and developer setpoint temperature.
The operations and functions of automatic film processors are handled under control of electronic circuitry, including a microprocessor connected to various process sensors and subsidiary controls to receive and dispense electronic signals in accordance with predefined software program instructions. Examples of such control circuitry are shown in U.S. Pat. No. 4,300,828 and in U.S. patent application Ser. No. 07/494,647, the disclosures of both of which are incorporated herein by reference.
If film is run through a processor at system start-up or during a change of mode, before the chemistry temperature has reached the designated setpoint setting for the selected mode, the image development may well be of substandard quality and, in worst case, not readable at all. For diagnostic imaging, this may necessitate retake with consequential patient inconvenience and additional radiation exposure. In cases of radiographic imaging utilized for progress monitoring purposes during a surgical operating procedure, this may lead to other undesirable consequences. It is, therefore, desirable to be able to prevent processing of exposed photosensitive media until setpoint temperatures are reached. This may be accomplished by configuring the temperature control circuitry to indicate a "ready" condition only when the developer, and optionally the fixer, chemicals reach their desired operating temperatures (i.e, until they are within X. of their setpoint temperatures). U.S. patent application Ser. No. 07/494,647 describes a system whereby the film drive transport mechanism is disabled to prevent the introduction of fresh film, until desired chemical temperatures are attained.
It is also desirable to be able to indicate a failure of the temperature control system. This is done conventionally by establishing an upper limit value, above which chemistry temperature would not normally be expected to go. This has the advantage of indicating an unacceptable overtemperature condition once setpoint temperature is reached, but provides no indication of improper operation prior to reaching setpoint. If the heat gain per unit time is too low, setpoint temperature may never be reached.
U.S. patent application Ser. No. 07/738,664, entitled "Method and Apparatus for Out-of-Rate Error Detection In Film Processor Temperature Control System filed Jul. 31, 1991 describes a processor temperature control system in which malfunctions in operation of heating and cooling cycles are determined utilizing comparisons of actual and normal rates of change in chemical or dryer air temperature over time. Failures are indicated based on comparisons of time variations in measured actual temperatures for a given heating (or cooling) cycle, with expected variations for the same cycle assuming normal rates of heating (or cooling) under normal temperature control system operating conditions. If the actual rate of measured temperature increase (or decrease) deviates by more than a preestablished acceptable tolerance from the expected normal rate of increase (or decrease), an error is indicated. The system can be set to shut down the processor or disable the film drive transport mechanism (with user-controllable override) to prevent the introduction of fresh film, if the error is not corrected. Such rate error detection scheme enables the rapid determination of temperature control system malfunction, prior to attainment of setpoint temperatures and flags errors which conventional error detection means would miss.
Regardless of the procedures employed for operational control or error diagnosis, processor temperature control systems suffer from the random occurrence of invalid actual temperature measurement data due to electrical noise or similar transients. This can interfere with normal temperature control functioning as, for example, by causing false starts of heating or cooling cycles, which themselves then result in unnecessary departures from equilibrium that have to be corrected. Wrong data can also cause false error designations leading to unnecessary lockouts or shutdowns or, at a minimum, to user annoyance.